1UNALIGNED MEMORY ACCESSES
2=========================
3 4Linux runs on a wide variety of architectures which have varying behaviour
5when it comes to memory access. This document presents some details about
6unaligned accesses, why you need to write code that doesn't cause them,
7and how to write such code!
8 9 10The definition of an unaligned access
11=====================================
12 13Unaligned memory accesses occur when you try to read N bytes of data starting
14from an address that is not evenly divisible by N (i.e. addr % N != 0).
15For example, reading 4 bytes of data from address 0x10004 is fine, but
16reading 4 bytes of data from address 0x10005 would be an unaligned memory
17access.
18 19The above may seem a little vague, as memory access can happen in different
20ways. The context here is at the machine code level: certain instructions read
21or write a number of bytes to or from memory (e.g. movb, movw, movl in x86
22assembly). As will become clear, it is relatively easy to spot C statements
23which will compile to multiple-byte memory access instructions, namely when
24dealing with types such as u16, u32 and u64.
25 26 27Natural alignment
28=================
29 30The rule mentioned above forms what we refer to as natural alignment:
31When accessing N bytes of memory, the base memory address must be evenly
32divisible by N, i.e. addr % N == 0.
33 34When writing code, assume the target architecture has natural alignment
35requirements.
36 37In reality, only a few architectures require natural alignment on all sizes
38of memory access. However, we must consider ALL supported architectures;
39writing code that satisfies natural alignment requirements is the easiest way
40to achieve full portability.
41 42 43Why unaligned access is bad
44===========================
45 46The effects of performing an unaligned memory access vary from architecture
47to architecture. It would be easy to write a whole document on the differences
48here; a summary of the common scenarios is presented below:
49 50 - Some architectures are able to perform unaligned memory accesses
51 transparently, but there is usually a significant performance cost.
52 - Some architectures raise processor exceptions when unaligned accesses
53 happen. The exception handler is able to correct the unaligned access,
54 at significant cost to performance.
55 - Some architectures raise processor exceptions when unaligned accesses
56 happen, but the exceptions do not contain enough information for the
57 unaligned access to be corrected.
58 - Some architectures are not capable of unaligned memory access, but will
59 silently perform a different memory access to the one that was requested,
60 resulting in a subtle code bug that is hard to detect!
61 62It should be obvious from the above that if your code causes unaligned
63memory accesses to happen, your code will not work correctly on certain
64platforms and will cause performance problems on others.
65 66 67Code that does not cause unaligned access
68=========================================
69 70At first, the concepts above may seem a little hard to relate to actual
71coding practice. After all, you don't have a great deal of control over
72memory addresses of certain variables, etc.
73 74Fortunately things are not too complex, as in most cases, the compiler
75ensures that things will work for you. For example, take the following
76structure:
77 78 struct foo {
79 u16 field1;
80 u32 field2;
81 u8 field3;
82 };
83 84Let us assume that an instance of the above structure resides in memory
85starting at address 0x10000. With a basic level of understanding, it would
86not be unreasonable to expect that accessing field2 would cause an unaligned
87access. You'd be expecting field2 to be located at offset 2 bytes into the
88structure, i.e. address 0x10002, but that address is not evenly divisible
89by 4 (remember, we're reading a 4 byte value here).
90 91Fortunately, the compiler understands the alignment constraints, so in the
92above case it would insert 2 bytes of padding in between field1 and field2.
93Therefore, for standard structure types you can always rely on the compiler
94to pad structures so that accesses to fields are suitably aligned (assuming
95you do not cast the field to a type of different length).
96 97Similarly, you can also rely on the compiler to align variables and function
98parameters to a naturally aligned scheme, based on the size of the type of
99the variable.
100 101At this point, it should be clear that accessing a single byte (u8 or char)
102will never cause an unaligned access, because all memory addresses are evenly
103divisible by one.
104 105On a related topic, with the above considerations in mind you may observe
106that you could reorder the fields in the structure in order to place fields
107where padding would otherwise be inserted, and hence reduce the overall
108resident memory size of structure instances. The optimal layout of the
109above example is:
110 111 struct foo {
112 u32 field2;
113 u16 field1;
114 u8 field3;
115 };
116 117For a natural alignment scheme, the compiler would only have to add a single
118byte of padding at the end of the structure. This padding is added in order
119to satisfy alignment constraints for arrays of these structures.
120 121Another point worth mentioning is the use of __attribute__((packed)) on a
122structure type. This GCC-specific attribute tells the compiler never to
123insert any padding within structures, useful when you want to use a C struct
124to represent some data that comes in a fixed arrangement 'off the wire'.
125 126You might be inclined to believe that usage of this attribute can easily
127lead to unaligned accesses when accessing fields that do not satisfy
128architectural alignment requirements. However, again, the compiler is aware
129of the alignment constraints and will generate extra instructions to perform
130the memory access in a way that does not cause unaligned access. Of course,
131the extra instructions obviously cause a loss in performance compared to the
132non-packed case, so the packed attribute should only be used when avoiding
133structure padding is of importance.
134 135 136Code that causes unaligned access
137=================================
138 139With the above in mind, let's move onto a real life example of a function
140that can cause an unaligned memory access. The following function adapted
141from include/linux/etherdevice.h is an optimized routine to compare two
142ethernet MAC addresses for equality.
143 144unsigned int compare_ether_addr(const u8 *addr1, const u8 *addr2)
145{
146 const u16 *a = (const u16 *) addr1;
147 const u16 *b = (const u16 *) addr2;
148 return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0;
149}
150 151In the above function, the reference to a[0] causes 2 bytes (16 bits) to
152be read from memory starting at address addr1. Think about what would happen
153if addr1 was an odd address such as 0x10003. (Hint: it'd be an unaligned
154access.)
155 156Despite the potential unaligned access problems with the above function, it
157is included in the kernel anyway but is understood to only work on
15816-bit-aligned addresses. It is up to the caller to ensure this alignment or
159not use this function at all. This alignment-unsafe function is still useful
160as it is a decent optimization for the cases when you can ensure alignment,
161which is true almost all of the time in ethernet networking context.
162 163 164Here is another example of some code that could cause unaligned accesses:
165 void myfunc(u8 *data, u32 value)
166 {
167 [...]
168 *((u32 *) data) = cpu_to_le32(value);
169 [...]
170 }
171 172This code will cause unaligned accesses every time the data parameter points
173to an address that is not evenly divisible by 4.
174 175In summary, the 2 main scenarios where you may run into unaligned access
176problems involve:
177 1. Casting variables to types of different lengths
178 2. Pointer arithmetic followed by access to at least 2 bytes of data
179 180 181Avoiding unaligned accesses
182===========================
183 184The easiest way to avoid unaligned access is to use the get_unaligned() and
185put_unaligned() macros provided by the <asm/unaligned.h> header file.
186 187Going back to an earlier example of code that potentially causes unaligned
188access:
189 190 void myfunc(u8 *data, u32 value)
191 {
192 [...]
193 *((u32 *) data) = cpu_to_le32(value);
194 [...]
195 }
196 197To avoid the unaligned memory access, you would rewrite it as follows:
198 199 void myfunc(u8 *data, u32 value)
200 {
201 [...]
202 value = cpu_to_le32(value);
203 put_unaligned(value, (u32 *) data);
204 [...]
205 }
206 207The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
208memory and you wish to avoid unaligned access, its usage is as follows:
209 210 u32 value = get_unaligned((u32 *) data);
211 212These macros work for memory accesses of any length (not just 32 bits as
213in the examples above). Be aware that when compared to standard access of
214aligned memory, using these macros to access unaligned memory can be costly in
215terms of performance.
216 217If use of such macros is not convenient, another option is to use memcpy(),
218where the source or destination (or both) are of type u8* or unsigned char*.
219Due to the byte-wise nature of this operation, unaligned accesses are avoided.
220 221 222Alignment vs. Networking
223========================
224 225On architectures that require aligned loads, networking requires that the IP
226header is aligned on a four-byte boundary to optimise the IP stack. For
227regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
228architectures this constant has the value 2 because the normal ethernet
229header is 14 bytes long, so in order to get proper alignment one needs to
230DMA to an address which can be expressed as 4*n + 2. One notable exception
231here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
232addresses can be very expensive and dwarf the cost of unaligned loads.
233 234For some ethernet hardware that cannot DMA to unaligned addresses like
2354*n+2 or non-ethernet hardware, this can be a problem, and it is then
236required to copy the incoming frame into an aligned buffer. Because this is
237unnecessary on architectures that can do unaligned accesses, the code can be
238made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so:
239 240#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
241 skb = original skb
242#else
243 skb = copy skb
244#endif
245 246--
247Authors: Daniel Drake <dsd@gentoo.org>,
248 Johannes Berg <johannes@sipsolutions.net>
249With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
250Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
251Vadim Lobanov
252 253